Segment Information - Segment Revenues and Earnings (Detail) - USD ($) $ in Millions	3 Months Ended								9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2019	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 236.3	$ 88.2	$ 65.3	$ 38.5	$ 32.2	$ 23.6	$ 22.6		$ 78.3	$ 83.5	$ 428.3	$ 114.3
Cost of Goods and Services Sold									49.5	90.1	362.7	112.6
Selling, General and Administrative Expense									19.4	51.2	28.9	75.3
Equity in earnings of unconsolidated ventures									(4.8)	(10.9)	(0.5)	(17.8)
Gain of sale of assets									(27.8)	(113.4)	(3.0)	(113.4)
Interest expense									(3.7)	(6.4)	0.0	(8.5)
Loss on extinguishment of debt									0.0		0.0	0.6
Interest and other income									(6.4)	(2.4)	(5.5)	(3.6)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	16.0	8.4	16.4	4.9	29.6	10.4	4.7		44.7	62.5	45.7	52.1
Less: Net income attributable to noncontrolling interest	0.1	$ 0.0	$ (2.7)	$ (0.6)	$ (0.3)	$ (0.9)	$ (0.1)		(1.2)	$ (0.1)	(3.3)	(2.0)
Income (loss) before taxes from continuing operations attributable to Forestar Group Inc.									43.5			50.1
Total assets	1,455.7								893.1		1,455.7
Investment in unconsolidated ventures	$ 7.3								11.7		$ 7.3
Operating Segments | Real Estate
Segment Reporting Information [Line Items]
Revenues									78.3			112.7
Cost of Goods and Services Sold									48.9			67.8
Selling, General and Administrative Expense									7.1			15.9
Equity in earnings of unconsolidated ventures									(4.8)			(16.4)
Gain of sale of assets									(18.6)			(1.9)
Interest expense									0.0			0.0
Loss on extinguishment of debt												0.0
Interest and other income									(1.8)			(2.0)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									47.5			49.3
Less: Net income attributable to noncontrolling interest									(1.2)			(2.0)
Income (loss) before taxes from continuing operations attributable to Forestar Group Inc.									46.3			47.3
Total assets									522.1
Investment in unconsolidated ventures									11.7
Operating Segments | Mineral [Member]
Segment Reporting Information [Line Items]
Revenues												1.5
Cost of Goods and Services Sold												38.3
Selling, General and Administrative Expense												1.4
Equity in earnings of unconsolidated ventures												(1.4)
Gain of sale of assets												(82.4)
Interest expense												0.0
Loss on extinguishment of debt												0.0
Interest and other income												0.0
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest												45.6
Less: Net income attributable to noncontrolling interest												0.0
Income (loss) before taxes from continuing operations attributable to Forestar Group Inc.												45.6
Operating Segments | Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues									0.0			0.1
Cost of Goods and Services Sold									0.6			6.5
Selling, General and Administrative Expense									0.3			0.4
Equity in earnings of unconsolidated ventures									0.0			0.0
Gain of sale of assets									(9.2)			(0.4)
Interest expense									0.0			0.0
Loss on extinguishment of debt												0.0
Interest and other income									0.0			0.0
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									8.3			(6.4)
Less: Net income attributable to noncontrolling interest									0.0			0.0
Income (loss) before taxes from continuing operations attributable to Forestar Group Inc.									8.3			(6.4)
Total assets									1.9
Investment in unconsolidated ventures									0.0
Items Not Allocated To Segments
Segment Reporting Information [Line Items]
Revenues									0.0			0.0
Cost of Goods and Services Sold									0.0			0.0
Selling, General and Administrative Expense									12.0			57.6
Equity in earnings of unconsolidated ventures									0.0			0.0
Gain of sale of assets									0.0			(28.7)
Interest expense									(3.7)			(8.5)
Loss on extinguishment of debt									0.0			(0.6)
Interest and other income									(4.6)			(1.6)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									(11.1)			(36.4)
Less: Net income attributable to noncontrolling interest									0.0			0.0
Income (loss) before taxes from continuing operations attributable to Forestar Group Inc.									(11.1)			(36.4)
Gain (Loss) on Sale of Properties									0.0			$ 28.7
Total assets									369.1
Investment in unconsolidated ventures									$ 0.0
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Oil and Gas Properties [Member]
Segment Reporting Information [Line Items]
Gain of sale of assets								$ (82.4)
Purchase price of properties sold								85.7
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Oil and Gas Properties [Member] | Continuing Operations
Segment Reporting Information [Line Items]
Goodwill, Impairment Loss								$ 37.9